Extracellular ATP and Plant Pathogen Defence

How plants defend themselves against pathogen attack is of intense research and commercial interest. We have identified an unexpected role of ATP in plant disease resistance, a finding that now enables us to discover previously unknown components of plant pathogen defence systems. ATP is a simple organic molecule used as a universal form of chemical energy for sustaining living organisms but, surprisingly, cells excrete it. In plants this excreted ATP is not wasted, but harnessed on the cell's surface to serve as a chemical switch that controls the plant's defences against attack by disease-causing microorganisms (pathogens). The switch is turned on or off by lowering or raising the levels of cell surface ATP, respectively. When turned on, the switch activates the cells to start producing compounds that either directly kill the invading pathogens or indirectly stop the infection. We discovered that this external ATP operates in partnership with salicylic acid, the parent compound of aspirin, which is a recognised natural plant chemical signal that immunises plants against infection by certain pathogens. Although located outside the cell, this switch system recruits other, as yet unknown, signalling components located both outside and inside the cell so as to control the expression of pathogen defence genes. In this work, we propose to identify the key signalling genes/proteins that enable this regulatory control system to work. We will use the model plant Arabidopsis thaliana and state-of-the-art technologies for identification of these genes and proteins. We will identify proteins that directly interact with ATP at the cell surface as well as the other proteins inside the cell that help to carry the signal to the nucleus to control defence gene expression. Overall, results from this study will increase our understanding of plant signalling systems and provide new opportunities for devising novel strategies to combat plant diseases.

We have discovered that ATP secreted to the extracellular matrix of plant cells functions as a molecular switch controlling pathogen defence response and disease resistance. Altering the levels of this extracellular ATP (eATP) modulates plant responses to pathogens i.e., increasing eATP by addition of exogenous ATP increases susceptibility to pathogens while ATP traps that specifically reduce eATP availability activate defence genes and increases resistance to viral and bacterial pathogens. Pathogens capable of activating defence gene expression trigger a rapid depletion of eATP, while those unable to activate defence genes do not affect eATP. Similarly salicylic acid (SA), a major plant defensive signalling compound, activates defence via depletion of eATP. Blocking this depletion by addition of exogenous ATP inhibits SA-induced defence gene expression. The molecular basis for this regulatory role of eATP in pathogen defence is unknown. Elucidation of the key components in the eATP-mediated signalling network regulating defence is a first step in our long-term goal to gain mechanistic insight into how eATP functions. Therefore, the proposed research focuses on this. We will use a combination of Proteomic and Transcriptomic technologies to identify signalling proteins/genes. Targeting phosphoproteins and ATP-binding proteins located in the extracellular matrix will identify putative eATP receptors or effectors at the top of the signalling cascade. Proteins/genes whose response to SA is blocked by exogenous ATP will be identified as putative downstream signalling/regulatory components. A reverse genetic approach, utilising T-DNA knockout and RNAi knockdown plants, will form the basis for target validation by using pathogen assays and analyses of defence gene expression as a screen. Finally, we will use single, double, and triple gene knockout plants to characterise the regulatory role played by an eATP-regulated extracellular protein that we have already identified.